Many implanted medical devices are powered by electricity. Some devices, such as artificial pacemakers, may be battery-powered because their power requirements are comparatively low. Other devices require considerably more power and cannot be adequately supplied by a battery on a long-term basis. An implantable blood pump, for example, requires considerably more power than an ordinary pacemaker. A pacemaker rhythmically generates electrical impulses to stimulate the heart muscle, but the pacemaker does not perform any mechanical pumping action. An implantable blood pump, by contrast, mechanically assists the heart muscle to pump blood, and for this reason has a considerably greater power requirement.
Power requirements for a typical pacemaker may be on the order of 10 milliwatts, as compared with an implantable blood pump, which may have a power requirement one thousand times greater. Implantable electric storage batteries are unable to provide such power for long periods of time, and the frequent invasive surgery that would be required to replace the batteries makes this option undesirable. If the implantable batteries are rechargeable, then the batteries must be recharged from time to time, and power must in some way be delivered to recharge the batteries. Once again, use of invasive surgery to deliver the recharge is not a desirable alternative.
Ordinarily the power source for a high-power device must be external to the body. To deliver the power to the device, the power must somehow transit through the skin. Power may be delivered through the skin by a percutaneous wire, but this method has drawbacks. A wire penetrating the skin provides a source for infection. Moreover, there is a risk that a wire penetrating the skin may accidentally be torn out, which may cause loss of power to the device and trauma to the patient.
Another way to deliver power through the skin is by way of induction. Two coils of electrically conductive wire, a primary coil external to the skin and a secondary coil implanted within the patient, may be inductively coupled. By energizing the primary coil with a time-varying current, a time-varying magnetic flux is produced by the primary coil. If the secondary coil is in proximity to the primary coil and is appropriately oriented, the time-varying magnetic flux will induce a time-varying current within the secondary coil, according to the principle of mutual induction. Power may be delivered through the skin using mutual induction. Systems delivering energy or power in this way are often called transcutaneous energy transfer, or TET, systems, sometimes referred to as TETS.
Mutual induction is a consequence of Faraday's Law of Induction. Faraday's Law holds that the electromotive force (emf) in a conducting coil of N turns and the rate of change of magnetic flux through the coil are related. Faraday's Law is embodied within the equation E=-N(d.PHI./dt), where E is the induced emf, N is the number of turns in the coil, and (d.PHI./dt) represents the change in magnetic flux with respect to time. The negative sign is a matter of convention, and is indicative of the direction of the induced emf.
Both the current in the primary coil and the current induced in the secondary coil are time-varying. Direct current (dc) will not result in a time-varying magnetic flux, and therefore will not create mutual induction. If the device being powered operates using dc, the current or voltage in the secondary coil must be conditioned for use by the device. Ordinarily, conditioning includes such functions as rectifying the current or voltage, filtering it to remove high-frequency components, and regulating it to provide substantially constant amounts of current or voltage.